So what are some genuine Class A amplifiers?

[teemuk]

To be more specific, class-AB does not "transition" to class-A. How they operate is, by definition, class-AB and although output devices conduct the whole signal swing at lower output levels the bias points are entirely different than in true a class-A amplifier.

Also, let it be mentioned that class-A push-pull amps are not void of crossover distortion that occurs when output is overdriven severily, (which is the main cause of audible crossover distortion in a reasonably well-designed class-AB amp).

 

[kosmicwizard]

Fender tweed champ is a great example of an open ended design it's 5 watts. Vintage Vox AC15 are supposed to be Class A as well. After that push-pull is more efficient, but supposedly many of the even ordered harmonics are lost with an AB power section.

 

[teemuk]

Was the SWR Interstellar Overdrive class A?

I believe it was class-AB.

 

[7bridges]

I'm pretty sure my Pass Labs XA100.8 amps are class A biased. They do run hot!

 

[Spupilup]

There are Class A tube and transistorized amplifiers. With an efficiency in the 25-30% range, they give off a lot of energy as heat. I’m surprised that California hasn’t banned them.

The best Class A power amps that I’ve heard are low power, 300B based. The hi-fi world loves Class A.

Class A is also used in some transmitting equipment, VHF is one example.

As I said above, my Wavelength mono amps (8 watts per channel, Westinghouse 300B’s) was one of the finest audio amps I’ve owned in over 40 years stricken with the disease of being an audiophile.

Klipsch designed his massive Klipschorns to be drive by 2a3 tubes pumping out under 3 watts. Cabinet efficiency is essential for all of these audiophile single ended triode amps. That’s why those older Tannoys were so popular as well as the Lowher concentric speaker design.

While I feel single ended triode amps could work well in small, efficient guitar amps, triodes IIRC tend to be a little soft as frequencies drop so, inherently, they may not represent an ideal choice of tube for a bass amp.

Now how about an OTL amp for bass. An output transformerless design would/could be interesting for a bass. I guess you could use the light and heat generated from 20 or 30 power tubes to illuminate the stage while heating the club.

Atma Sphere - About us

 

[Jeff Scott]

I got curious about the mystical class A guitar amplifier, and wondered if there was a bass equivalent.

Guitarists need mystical things to happen to their tone. Bassists are a far more practical lot.

Crazy inefficient. Unless you're hungry.

Or, cold.

Was the SWR Interstellar Overdrive class A? That was another unit sold as a preamp but had a speaker output.

View attachment 3966761

The preamp section is Class A, the tiny power amp section is not, it's Class A/B

 

[micguy]

If you want the same overall effect as a class A amplifier, my suggestion is to run a well designed class AB amplifier and a space heater. The amp will sound pretty much the same, and you'll get the same warm feeling.

 

[Spupilup]

Atma-Sphere MA-3

Something like this 500 watt OTL amp must be as difficult to keep running smoothly as a carbureted Jag V12!!!

 

[PennyroyalWe]

I’ve only ever seen class A in very small guitar amps. As I understand it class A has the power tubes at 100% and the volume knob adjusts the preamp tube, which is why it’s usually reserved for low watt applications. I could be wrong though.

 

[dbase]

I'm pretty sure my Pass Labs XA100.8 amps are class A biased. They do run hot!

View attachment 3966873

Those power cords look quite hefty

 

[agedhorse]

I'd venture to say that if you're not finding any examples its for good reason. Amp designers need to design amps that sell. If no one wants an inefficient heat sink that makes a bit of noise, then no one will design and build one.

Yes, the reasons are good why they are neither common nor popular.

Well implemeted class A/B amplifiers will for use with bass guitar probably behave very, very similarly.
The very small amount of transient driven crossover distortion would probably be "masked" by the somewhat "chaotic" transients from a bass guitar. That especially on stage, where higher power is needed. I think Yamaha 2200/2201 is a good example of an amp that has very low distortion (typically 0.005%, medium power mid band and 0,01% THD, full continuous power, mid band). And that is a design that is about 40 years old.

While I have not seen class D amps with such impressive numbers, but modern class A/B, or H amps might do it too.

Class D amps can and do achieve these specs also.

Nelson Pass has built and published plans for many different Class-A transistor amps for home hi-fi. I got to listen to a 30 watt per channel Pass Labs Aleph 3 once and probably the strongest impression it left on me is it was it was the only audio equipment I'd tried which might literally be able to cook an egg and boil water; it was a lot hotter than what I'm accustomed to considering as "hot" in home or pro amplification. It was explained to me once that Class-A basically means running at full power all the time, anything not used for signal is lost as heat. Crazy inefficient. Unless you're hungry.

Yes, they run very hot. Nelson (and others) designed a sliding class A bias circuit that improved on this somewhat.

I went to school at the same university that Nelson did, when he finished, I took over the technician job he had. He was a very clever guy.

I’ve only ever seen class A in very small guitar amps. As I understand it class A has the power tubes at 100% and the volume knob adjusts the preamp tube, which is why it’s usually reserved for low watt applications. I could be wrong though.

Yup, in this case you are wrong. Class A describes the conduction angle that the transistor operates through. Class A transistors operate through a 360 degree conduction angle, class AB operates 180-360 degrees, class B operates at 180 degrees and class C (not useful for audio) operates below 180 degrees.

 

[solodb]

Is the Fender Champ 12 class A ? It has one 6l6 output tube and 2 12ax7 preamp tubes. It sounds pretty good for bass at low practicing levels and has a line level out and a headphone jack which can mute the 12" guitar focused speaker. I have been using it as a powered pre-amp sort of ?

 

[agedhorse]

Or this one:

http://www.accuphase.com/technical_information/a-250_technical_information.pdf

Did you happen to notice the weight and the quiescent power draw? It's not going to be operating in class A mode above say 300 watts into lower impedance loads either but MAY operate in class A up to rated power into 8 ohms.

This amp uses 2 x C19 AC power inlets on different branch circuits!!!

 

[nilorius]

If there where some A class amps at '70 then now they are absolutely C.

 

[teemuk]

Now how about an OTL amp for bass. An output transformerless design would/could be interesting for a bass.

Today we have these things called "MOSFETs", which by their transfer characteristics are nearly identical to pentode tubes, yet do not require filaments and provide lower internal impedance and thus better match to low impedance of generic loudspeakers.

Essentially, we already have "OTL" amps. Moscode hifi amp, and its musical instrument amp versions like solid-state Ampeg SVT-Pro series or Randall Dynavalve (iirc?), are essentially a modern rendition of Futterman's classic OTL amp: single-ended triode class-A driver (Futterman's ideal of ultimate tube sound) buffered by transparent output "power" buffer. Even Futterman regarded MOSFETs as the ultimate solution that would obsolete tubes in the latter.

 

[el murdoque]

Reußenzehn The Bass Studio Amplifier

The Reussenzehn The Bass Studio Amplifier is a single sided class A amplifier.

 

[sawzalot]

For the power levels required for class A power amps to make bass audible in a loud band I think you'd need a forklift to move the amps, a power distro wired into the venue's main breaker panel to power them, plus HVAC cooling units to keep the venue at a reasonable temperature. I'll keep my class D amps .

 

[lucas303]

I'm pretty sure my Pass Labs XA100.8 amps are class A biased. They do run hot!

View attachment 3966873

What, no cable risers?

But seriously, that's a nice looking system. Mine isn't as pretty, but sounds divine.

 

[74hc]

Some one over at AX84.com made one of the SE amps into a bass amp. Clips sounded good, and he posted the changes. I've not gotten around to making one of those, but I've been meaning to in the 10 years or so. No, I don't procrastinate.

 

[Spupilup]

Today we have these things called "MOSFETs", which by their transfer characteristics are nearly identical to pentode tubes, yet do not require filaments and provide lower internal impedance and thus better match to low impedance of generic loudspeakers.

Essentially, we already have "OTL" amps. Moscode hifi amp, and its musical instrument amp versions like solid-state Ampeg SVT-Pro series or Randall Dynavalve (iirc?), are essentially a modern rendition of Futterman's classic OTL amp: single-ended triode class-A driver (Futterman's ideal of ultimate tube sound) buffered by transparent output "power" buffer. Even Futterman regarded MOSFETs as the ultimate solution that would obsolete tubes in the latter.

Wasn't he behind the NYAL amps from the early 80's? They tended to self destruct regularly, IIRC. They were beautiful sounding amps but short on reliability.